In the process of manufacturing semiconductor devices, LCD panels, and others, there are hundreds of processing equipments and thus hundreds of manufacturing steps. It is very difficult for the flow of the wafers, flat panels, or LCDs (hereafter workpiece) to be uniform from step to step, from tool to tool. Despite the best planners, there is always the unexpected scenario, such as a tool down, an emergency lot coming through, a periodic maintenance lasting longer than planned, thus there are various accumulations of the workpieces at certain steps for certain tools. The accumulated workpieces will need to be stored in a storage stocker, waiting to be processed.
In a typical bare stocker system, a robot is typically used to remove the workpieces from the carrier boxes, and then loaded into a storage chamber, where the workpieces are stored without the original carrier boxes. For box stocker system, the workpieces are stored together with the carrier boxes, without the need for removing them out of the carrier boxes.
The carrier box is a protective container to minimize the substrate exposure to the environment outside of the processing machines and protect the substrate against particulate contamination. The carrier boxes are handled by an operator or by an automatic material handling system such as automatically guided or overhead transport vehicles that travel on predetermined routes, either on the ground or suspended on ceiling tracks. For semiconductor wafers, the carrier boxes are normally cassettes pods, such as SMIF (standard machine interface) or FOUP (front opening unified pod), which are handled by an operator at the tools equipment front end module (EFEM) or automatically picked up and placed in the automatic transport system.
One type of conventional transport system is an overhead transport (OHT) system, which comprises an OHT vehicle, which runs freely on a rail mounted on a ceiling. The OHT vehicle carries a cassette pod between facility equipment, such as processing systems and stockers. The OHT vehicle can load or unload a cassette pod onto a load port of the equipment, for example a MLP (Mobile Launch Platform) or an EFEM. From there, the cassette pod or the wafers can be transferred from or to the inside of the equipment.